Under growing pressure over recently disclosed surveillance programs, the head of the National Security Agency told lawmakers Tuesday that gathering telephone data and monitoring Internet use has helped to disrupt more than 50 "potential terrorist events" since the attacks of Sept. 11, 2001.

Gen. Keith Alexander and other officials, testifying before a largely sympathetic House Intelligence Committee, identified two new cases — an alleged plot to blow up the New York Stock Exchange and a U.S. resident who helped finance a terrorist group in Somalia — that they said proved the worth of the surveillance programs disclosed this month by former Marylander Edward Snowden.

The programs "are critical to ... our nation and our allies' security," said Alexander, head of the NSA and U.S. Cyber Command. "They assist the intelligence community efforts to connect the dots."

Most of the plots were foiled by surveillance of foreigners overseas, the kind of spying the NSA has done since it was created in 1952 to monitor communications and other signals intelligence.

Alexander said 10 of the 50 cases might have involved domestic telephone records, but could not say how many actually did.

Civil liberties activists and some members of Congress have called the collection of "telephony metadata" — details of numbers dialed and lengths of calls, though not what was said — the most worrisome part of the NSA operation, and questioned whether it violates Americans' privacy.

Rep. C.A. Dutch Ruppersberger, the top Democrat on the Intelligence Committee, says he understands those concerns. "To be honest with you, if I weren't in this field, at first blush, I'd say, 'What is this?'" the Baltimore County lawmaker said after the hearing. "That's why we felt very strongly we needed to have these open hearings."

Ruppersberger said he has asked Alexander and others in the intelligence community "to try to declassify as much as you can without violating national security and giving our enemies, especially al-Qaida, more information than they have already because of the leaks."

The hearing came amid opinion polls this week that indicated that a plurality of Americans believe the public was served by the leaks, and half no long consider President Barack Obama honest and trustworthy. Fifty-four percent of Americans believe the person responsible for the leaks should be prosecuted, according to a USA Today/Pew Research Center Poll released Monday. But the same survey indicated that 49 percent believe the release of classified information serves rather than harms the public interest.

Snowden, 29, a former computer systems administrator at an NSA facility outside Honolulu, disclosed details of the telephone and Internet programs to the British newspaper The Guardian and The Washington Post.

He came to Maryland with his family in 1999, lived in Crofton and Ellicott City, attended schools in Anne Arundel County and took classes at Anne Arundel Community College.

Alexander said Snowden worked at the NSA for 15 months as an outside contractor. Snowden was an employee of Booz Allen Hamilton, a consulting firm, for three months this year, the company has said. Snowden worked in 2012 for Dell, according to a government form he signed last year.

Snowden apparently used his access at the NSA to download top-secret documents onto a portable data storage device and spirit them out of the facility.

In some quarters, he is viewed as a hero, with praise from personalities as disparate as Michael Moore and Glen Beck, Al Gore and Ron Paul. More than 84,000 have signed a petition on the White House website demanding that he be pardoned for any crimes he might have committed related to revealing the secret programs, though he has not been charged.

Ruppersberger called Snowden a felon who "has turned his back on his country and done tremendous damage not only to the United States but to our allies."

Ruppersberger said the disclosures "will cost lives in the short term and in the long term." He said he had told Alexander that "we need an alarm system so that if someone decides to do this, they can't get access to that volume of information" that Snowden appears to have accessed.

Alexander said the NSA was considering creating a "two-person control" system to prevent anyone from downloading sensitive data without a partner approving it. He also said he would give the House and Senate intelligence committees a classified list of the plots thwarted by the NSA telephone and Internet programs that Snowden disclosed.

The highly secretive NSA, which is fenced off from the rest of Fort Meade, collects and processes intelligence gathered from phone calls, Internet activity and other electronic communications.

The officials said the agency's archive of domestic telephone calling records is used only for terrorism investigations and espionage cases. Only 22 analysts of the agency's vast workforce are authorized to access the database, and they conducted about 300 queries last year.

Deputy FBI Director Sean Joyce told the Intelligence Committee of two terrorism cases that he said the NSA surveillance efforts had helped disrupt.

NSA monitoring of an extremist in Yemen led to his communications with Khalid Ouazzani of Kansas City and an alleged plot to bomb the New York Stock exchange, Joyce said. Ouazzani pleaded guilty in 2010 to providing money to al-Qaida, and to related charges of money laundering and bank fraud.

In another case, Joyce said, the NSA "provided us a telephone number only in San Diego that had indirect contact with an extremist outside the United States." Further investigation led the FBI to uncover a terrorist financing plot in Somalia.

Basaaly Saeed Moalin, a cabdriver in San Diego, and three others were convicted this year of conspiracy to provide material support to a foreign terrorist organization and money-laundering conspiracy.

Previously, officials had said the NSA surveillance played a role in the 2009 arrests of Najibullah Zazi, an Afghan-American in Denver who later pleaded guilty to planning suicide bombings in the New York subway system, and David Headley, a Pakistani-American from Chicago who was convicted this year for his role in the 2008 bombings in Mumbai, India, which killed 166 people.

After the hearing, Alexander said Snowden had gained access to the most sensitive document he leaked, an order marked "Top Secret" from the Foreign Intelligence Surveillance Court, during his NSA orientation.

The warrant "was on a web server that he as an analyst [had access to] coming into the Threat Operations Center," Alexander said. "It was in a special classified section that, as he was getting his training, he went to."

It wasn't immediately clear whether Snowden copied the document during his training or later.